Enhancement of signal quality in esophageal recordings of diaphragm EMG

Sinderby, Christer A., Jennifer C. Beck, Lars H. Lindström, and Alejandro E. Grassino. Enhancement of signalquality in esophageal recordings of diaphragm EMG. J. Appl. Physiol. 82(4): 1370-1377, 1997.The cruraldiaphragm electromyogram (EMGdi) is recorded from a sheet of muscle,the fiber direction of which is mostly perpendicular to an esophagealbipolar electrode. The region from which the action potentials areelicited, the electrically active region of the diaphragm(EAR_di) and the center of this region (EAR_{di ctr}) mayvary during voluntary contractions in terms of their position withrespect to an esophageal electrode. Depending on the bipolarelectrode's position with respect to theEAR_{di ctr}, the EMGdi isfiltered to different degrees. The objectives of the present study wereto reduce these filtering effects on the EMGdi by developing ananalysis algorithm referred to as the "double-subtraction technique." The results showed that changes in the position of theEAR_{di ctr} by ±5 mm withrespect to the electrode pairs located 10 mm caudal and 10 mm cephaladprovided a systematic variation in the EMG power spectrumcenter-frequency values by ±10%. The double-subtraction techniquereduced the influence of movement of theEAR_{di ctr} relative to theelectrode array on EMG power spectrum center frequency and root meansquare values, increased the signal-to-noise ratio by 2 dB, andincreased the number of EMG samples that were accepted by the signalquality indexes by 50%.